1. Field of the Invention
This invention relates to focus evaluation test stations for characterizing position error in an EO sensor that does not possess dynamic focusing capability, and more particularly to a motionless test station that does not translate the source and target to vary the apparent axial EO Sensor image position.
2. Description of the Related Art
A particular class of Electro-Optical (EO) sensors comprises an optical assembly that focuses collimated electromagnetic (EM) radiation onto a detector. The detector is nominally positioned at a known desired position. Typically, the detector is positioned at the focal plane of the optical assembly for optimum focus if the system is set for infinity focus. However, it may be desirable to position the detector elsewhere. This class of EO sensor has no dynamic focusing capability to adjust the position of the detector. The detector is responsive to wavelengths of EM radiation that lie in a range between 0.2 to 30 microns. A detector may, for example, cover visible 0.4-0.7 um, NIR 0.7-1.1 um, SWIR 1.1-2.5 um, MWIR 3-5 um or LWIR 8-14 um.
During assembly of the EO sensor, the detector is placed at a known desired position relative to the optical assembly (e.g. the back focal plane of the optical assembly) within the mechanical tolerance of the assembly process. This tolerance measured as Δd between the desired detector position ddesired and the actual detector position dactual may be unacceptably large in terms of optical aberrations. In general, an EO sensor may be considered to be “imaging well” if the root-mean-square (RMS) error of the aberrations is less than or equal to ¼ wave. While not a strict metric, this rule of thumb insures that the point spread function retains its general shape. For military and high-end commercial EO sensors, it is important that the sensor is “imaging well” over a variety of temperature and vibration environments. Consequently, the EO sensor is tested to determine detector position error; the distance of actual detector position from the desired detector position. If the detector position error is not within the specific optical tolerance, the position of the detector may be changed by, for example, inserting mechanical “shims” into the assembly.
A focus evaluation test station 10 for measuring the detector position error Δd of an EO sensor 12 and an afocal optical system 14 formed by the test station and EO sensor are shown in FIGS. 1 and 2. The point spread function (PSF) describes the response of the imaging system to a point target. The degree of broadening (blurring) of the PSF is a measure of the quality of the imaging system. Other factors being equal, the blurring of the PSF will be minimum when the detector is positioned at the desired position.
Test station 10 comprises a fixture 16 for mounting the EO sensor 12 so that its optical axis is coincident with an optical axis 18 of the test station. A source 20 emits diverging EM radiation 21 along the optical axis 18. A target h 22 such as a single slit is positioned orthogonal to optical axis 18 at the front focal plane FF1 of collimating optics 24. Source 20 and target 22 are mounted on a linear translation stage 26 that moves parallel to optical axis 18 to move the position of target 22 by a distance ZA about its nominal position at the front focal plane FF1. Each position on target 22 is focused at the same image plane a distance Z′A from the rear focal plane F′R2 of the EO sensor's optical assembly 28 where the detector 30 is nominally positioned. The transverse magnification m (x-y plane orthogonal to optical axis 18) is defined by the ratio of the EO sensor back focal length and the collimating optics front focal length. The longitudinal magnification mz (along the optical axis 18) is defined as the square of the transverse magnification. The target h is magnified to a conjugate target h′=m*h and Z′A=mz*ZA=m2*ZA. A processor 32 receives images captured by detector 30 and processes them to determine the detector position error.
To test EO sensor 12 and measure its detector position error, linear translation stage 26 moves target h 22 to a known position ZA. Detector 30 captures an image of target h 22. Processor 32 computes, for example, a 2-d FFT of the image (or impulse response convolution). The processor samples the 2-D FFT orthogonally to the orientation of the slit to produce a Line Transfer Function (LTF). The LTF may be integrated over all spatial frequencies or sampled at a predetermined spatial frequency to produce a value that is recorded along with stage position ZA. Using the equations above for longitudinal magnification, the processor maps the stage position ZA to the image plane position Z′A. This process is repeated for multiple stage positions. The processor fits a through focus curve 34 to the raw measurements 36 of paired LTF values/image plane positions as shown in FIG. 3. Through focus curve 34 is typically fit with a quadratic or fourth order polynomial curve with the peak 38 denoting the detector plane position Z′A. In some cases, the through focus curve covers large amounts of defocus and is more accurately fit with a Gaussian functional form. In either case, the peak 38 corresponds to the value of Z′A for the actual position of the detector. The difference between the computed Z′A and the desired Z′A (usually at or near the rear focal plane of the EO Sensor 12, if the system is set for infinity focus) is the detector position Δd. This difference is equivalent to the shift of the peak from the desired detector position.